In recent years there has been an upsurge in interest in the production of both chemicals and transportation fuels from non-petroleum carbon sources such as methane, tar sands, oil shale and the like. This interest has focused for lack of good direct conversion processes on indirect processes, which often go through a synthesis gas intermediate with subsequent conversion of the synthesis gas via Fischer-Tropsch and related processes to hydrocarbons and/or oxygenates. Oxygenates, particularly lower alkanols, are common products of such synthesis gas reactions, and high conversion, selective processes to convert an alkanol or a mixture of alkanols to higher molecular weight alkanols have substantial commercial potential.
One potential process for alkanol feeds uses the well-known, non-catalytic Guerbet reaction which converts a lower molecular weight alkanol to a branched or linear higher molecular weight alkanol in the presence of an alkali metal alkoxide dissolved in the alkanol to be converted. Such processes are uncatalyzed, moderate temperature batch reactions. When considered for industrial use, however, the Guerbet reaction suffers an economic disadvantage in that a portion of the starting alkanol (and possibly some of the product) is consumed by oxidation to the corresponding carboxylic acid unless special agents are added. One publication suggests the use of a mixture of potassium hydroxide and boric oxide to suppress acid formation which is said to improve the yield.
More recently, an improved Guerbet reaction has been reported which uses a "catalyst" system employing magnesium oxide, potassium carbonate, and copper chromite for converting, for example, ethanol to higher alcohols including 1-butanol, and 1-butanol to higher alkanols including 2-ethyl-1-hexanol (J. Org. Chem. 22, 540-2 (1957)). The reaction is of the batch type and the "catalyst" is said to have limited lifetime.
Another improvement in the Guerbet reaction, discussed in J. Mol. Catalysis 33, 15-21 (1985), uses a sodium alkoxide mixed with 5 percent rhodium on alumina as a "catalyst." Mixtures of 1-butanol and methanol are said to be converted by the "catalyst" to a mixture of 2-ethyl-1-hexanol and 2-methyl- 1-butanol.
Still other batch Guerbet reaction variations include water removal to improve yield and the use of an alkali metal hydroxide "catalyst" (U.S. Pat. No. 3,328,470), the use of an alkali metal alcoholate/boric acid ester "catalyst" (U.S. Pat. No. 2,861,110), and the addition of a nickel "catalyst" to the metal alkoxide (J. Am. Chem. Soc. 7 & 52 (1953)).
Octane demand has continued to increase in recent years and growth is likely to continue in the United States. For example, it has been estimated that clear pool octane demand has been increasing by 0.15 units/year in recent years. Addition of alkanols and ethers such as methanol, ethanol and methyl t-butyl ether to gasoline to improve octane number and/or improve the effect of gasoline combustion in internal combustion engines on the environment has been the subject of a number of recent publications.
Methanol is generally made from synthesis gas and ethanol can be made by carbonylation of methanol or more usually from agricultural products by fermentation. Higher alkanols can also result from the catalyzed conversion of synthesis gas. Olefins such as ethylene and propylene are made in large quantities by the cracking of alkanes such as ethane, propane and naphtha. Potentially, additional large amounts of ethylene are available from natural gas by the oxidative coupling of the methane component.
Methanol, while effective if used essentially pure for transportation fuel, is not a good additive for gasoline and is also potentially available in large quantities by the partial oxidation the methane component in natural gas. Ethanol has shown promise as a gasoline additive, but isobutanol in particular is valuable as it can be dehydrated to isobutylene and reacted with methanol to form methyl t-butyl ether which is an excellent octane improver that can be easily blended into gasoline. Isobutanol is also an effective octane improver. The methyl ether of i-pentanol is also an excellent octane improver for gasoline. U.K. Patent Application GB 2,123,411 describes a process for making a mixture of octane improving ethers by synthesizing an alkanol mixture containing methanol, ethanol, and higher alkanols and dehydrating the higher alkanols and etherification.
Because of the large amount of methanol available and its problems as a gasoline additive, processes which convert methanol to effective gasoline additives are valuable. Well-known is the Mobil process for converting methanol to gasoline-range hydrocarbons over an aluminum-containing molecular sieve. Little work has been reported on effectively converting methanol to higher alcohols, in particular, i-butanol.
Now a process has been found which allows a continuous, vapor phase, catalytic Guerbet-type of condensation to be effected on a large variety of different alkanols, their dehydration products (alkenes), aldehydes, ethers and mixtures thereof. In particular, a continuous vapor phase process for direct condensation of methanol and/or dimethyl ether with dilute acetylene feedstream to form a mixture containing at least one higher molecular weight alkanol, such as n-propanol or i-butanol, over an alkaline catalyst which is, advantageously, an essentially magnesium oxide catalyst.